Gas chromatography (GC) is a useful technique for the separation and quantification of molecules. The detection and quantification of organic molecules from a GC is commonly carried out by flame ionization detection (FID) because of its high sensitivity to carbon. The FID operates by ionizing a fraction of the carbon containing molecules and measuring the resulting number of ions with a current collector. The fraction of ionized carbons depends on the number and nature of carbon atoms in a molecule, including the number and types of bonds of each carbon atom (e.g., carbonyl, aldehyde, ether). Methane (CH4) has the highest per carbon sensitivity in the FID because of its four bonds to hydrogen; carbons that contain double/triple bonds or bonds to elements other than H decrease the fraction of ionized carbons created in the FID, and thereby decrease the sensitivity of the FID for that molecule on a per carbon basis. The decreased sensitivity of the FID to various molecules requires that the response of the FID to each molecule be determined through laborious calibrations in order to accurately quantify the amount of a molecule in a sample. In some instances the sensitivity of the FID to the molecule is so low that it is effectively undetectable. These molecules usually have a large heteroatom content and include carbon monoxide (CO), carbon dioxide (CO2), carbon disulfide (CS2), carbonyl sulfide (COS), carbon tetrachloride (CCl4), hydrogen cyanide (HCN), formamide (CH3NO), formaldehyde (CH2O) and formic acid (CH2O2). The decrease in carbon sensitivity of non-methane molecules and the laborious requirements of calibration reduce the utility of the FID detector and the GC in general.
The chemical conversion of a molecule into methane after its separation in the GC, but before its detection in the FID, increases the sensitivity of the detector to the molecule and eliminates the need for the calibration of its relative response factor to methane because all molecules are detected as methane. The conversion of GC effluents to methane can be accomplished a variety of ways with varying results and ease.
In one such embodiment the GC column effluent is combusted to CO2 (and byproducts) and then reduced to CH4 (and byproducts) in two separate reaction vessels separated by a 4-port valve and tubing (T. Watanabe et al., Chromatography, 27 (2006), pp. 49-55; T. Watanabe et al., Talanta, 72 (2007), pp. 1655-1658). The combustion reaction utilizes a commercially available palladium-asbestos catalyst packed into a stainless steel tube containing quartz wool. The reduction reaction utilizes a commercially available nickel catalyst packed into a stainless steel tube containing quartz wool. This setup involves the flow control of oxygen and hydrogen streams into a mixing point before the catalytic reduction zone of the reactor leading to a possibly dangerous and explosive mixtures of gases. There are separate temperature controls and heating elements for the combustion and reduction chambers. The effluent of the reduction reactor is fed to the FID.
Another example comprises a similar sequential reaction system of stainless steel tubes utilizing commercially available catalysts consisting of 10% palladium on alumina for the combustion chamber, and nickel on diatomaceous earth catalyst for the reduction chamber (S. Maduskar et al., Lab on a Chip, 15 (2015), pp. 440-447).
In another embodiment, using a single reaction scheme, a commercially available Ni catalyst is packed into a stainless steel tube and heated to 375° C. with hydrogen to convert carbon monoxide (CO) and carbon dioxide (CO2) to methane, currently offered by Agilent Technologies. Another example of a CO/CO2 to methane conversion reactor is offered by SRI Instruments and involves a packed metal tube (jet) that is inserted directly into the FID and heated to 380° C. with hydrogen. These reactors are designed for the sole conversion of CO/CO2 to methane and Ni is easily poisoned by sulfur or excessive moisture or oxygen.